What is the format of the drive? The Philips HTS6510 can only read USB drives that are formatted as FAT16 or FAT32. It can't read files from an NTFS drive. NTFS is the "new file system" for Windows but dates to Windows NT/Windows 2K or so. This is the default format for larger capacity USB drives since more files can be added per folder. See page 51 of the manual for the Philips HTS6510 for the file format specifications for USB drives.
If the file format is FAT16 or FAT32, try a different USB drive and check the current drive in another device. The USB drive may be damaged or may have lost the data it needs to find the files. All flash memory can fail with no warning. Replace a faulty drive. Alternatively, the USB port on the HTS6510 could be damaged. This can happen if too much current flows through the port. If the port is damaged, I don't recommend having the unit repaired unless it is under warranty (1 year). Copy your files to a CD and listen to the music or view the images from that media. The overall cost will be lower.
Philips has the manual on the product support page: https://www.philips.co.uk/c-p/HTS6510_05/-/support
. The current link to the manual is https://www.download.p4c.philips.com/files/h/hts6510_05/hts6510_05_dfu_eng.pdf
I hope this helps.